The present invention relates generally to signal detection, and more specifically to apparatus and techniques employing quadrature correlation to detect signal pulses in the presence of random noise.
Signal pulses of a known carrier frequency and width are employed in a variety of situations for communication, object detection and/or object location, as well as other purposes. To the extent that the signal pulses remain relatively undistorted and of a significantly higher amplitude than any background noise, pulse detection can be accomplished with relative ease and simplicity. However, there are many situations in which it is necessary to reliably detect tone pulses in a signal contaminated with random noise of significant amplitude relative to the tone pulse amplitude, either because of substantial attenuation of the tone pulses over the signal transmission path or because of a high level of ambient noise from any of a variety of sources. In such situations, simple amplitude and/or frequency discrimination may not provide reliable tone pulse detection, and other more sophisticated detection techniques are required.
One known general technique for improving signal detection involves correlation of two signals, one of which may be a reference signal. Both analog and digital forms of signal correlation are known. For example, U.S. Pat. No. 3,346,862 issued to I. G. Raudsep on Oct. 10, 1967 discloses an analog autocorrelation system for determining the time difference between a pair of pulse signals of common origin. The system employs weighting filter means for modifying the power spectra of the pulse signals to optimize the autocorrelation function. U.S. Pat. No. 3,646,334 issued to Ivar Wold on Feb. 29, 1972 discloses a hybrid analog/digital system in which two input signals to be correlated are sampled, the samples of one of the signals inserted into a recirculating memory time compressor, the output of the memory multiplied with the other signal, and the product signal averaged to determine correlation of the input signals.
Other known refinements in correlation techniques involve multiplication of the input signal with each of quadrature components of a reference signal. The product signals are integrated with respect to time to produce real and imaginary components of correlation of the input and reference signals. The real and imaginary components are combined in accordance with the Pythagorean theorem to produce an indication of correlation of the signals. A variation of this method is embodied in a signal processor disclosed in U.S. Pat. No. 3,878,526 issued to N. E. Pedersen on Apr. 15, 1975. As is typical of traditional quadrature demodulation and signal detection systems, the Pedersen processor is an analog system.
It is known that digital systems in general have certain inherent advantages over analog systems. Some of these are set forth in U.S. Pat. No. 3,039,094 issued to V. C. Anderson on June 12, 1972 which discloses a digital system for beam steering of a fixed transducer array. One disclosed embodiment utilizes shift register memory apparatus to accomplish beam steering.
Among the general advantages set forth for digital systems are that the digital signals produced thereby and used therein are directly compatible with digital computers, which provide great flexibility and signal processing power. Also, the use of digital signal processing provides for a normalized output characterized by a true signal-to-noise ratio rather than proportionality to either signal or noise alone. This normalization reduces the dynamic range requirements of components and circuits used in the system, since variations in background noise do not change the reference noise output of the system. Further, digital systems are minimally susceptible to errors and changes in calibration caused by aging of components and changes in operating parameters.
As also discussed in U.S. pat. No. 3,039,094, in many situations the polarity of a band-limited signal contains nearly as much information as the complete analog signal itself. This principle may be advantageously applied by dividing an input analog signal of interest into two classes determined by its instantaneous polarity, and representing it by a time series consisting of two possible voltage states. This so called "clipped signal" may be simply and conveniently produced by a clipper or clipping amplifier. The clipping level may be set to a limit desirable for subsequent digital signal processing, and the voltage states assigned values of +1 and -1
U.S. pat. No. 3,039,094, further states that, "a band-limited signal may be represented by a sequence of individual amplitude samples providing the sampling rate is equal to or greater than twice the highest frequency in the signal". This principle and the principle of the preceding paragraph can be combined to permit the input signal to be represented by a set of binary digits, each sample having a value of +1 or -1, depending on the polarity of the input signal at the sampling instant.
The applicant has discovered that the advantages of quadrature correlation and digital signal processing utilizing clipped signals may be uniquely combined to provide a clipped quadrature correlation pulse detector and pulse detection technique. In addition to providing superior pulse detection capabilities this detector and detection technique have been found to possess characteristics and features which are particularly advantageous in making highly accurate and reliable phase determinations.